1. Field of the Invention
The present invention relates to a thick dielectric composition for a solid state display (SSD) and a method for fabricating the solid state display, and more particularly, to a composition whose thermal and electrical characteristics are improved and having a high permittivity and dielectric property, which is used for thick dielectrics of a solid state display, and a method for fabricating the solid state display by forming thick dielectrics on a rear substrate using the composition and thereafter simultaneously pressurizing and heating the same.
2. Description of the Background Art
Typical flat panel displays are generally classified into liquid crystal displays (LCD), field emission displays (FED), plasma display panels (PDP) and electroluminescence (EL) and so on.
Among the above-mentioned flat panel displays, the electroluminescence (EL) are active displays using the luminescence of a semiconductive fluorescent material by applying an electric field to the above fluorescent material applied to a glass substrate or transparent organic film, which are divided into thin film electroluminescent devices (TFEL) for displaying information in orange yellow, dispersion type electroluminescent devices (EL), and solid state displays (SSD).
Herein, the above solid state displays (SDD) are improvements on the above thin film electroluminescent devices (TFEL), and the derivation principle thereof will now be described in detail.
Generally, in an electroluminescent device, a rear electrode, first insulating layer, fluorescent layer, second insulating layer, and transparent electrode are stacked on a predetermined substrate one after another. When a voltage is applied between the rear electrode and the transparent electrode, visible rays are outputted from the fluorescent layer. At this time, the transparent layer is surrounded by an insulating material, which makes the fluorescent layer not broken down by an electric filed inputted from the areas excepting the fluorescent layer. That is, the insulating film formed on the fluorescent layer by the insulating material makes it possible to stably apply a high voltage to the fluorescent layer, and prevents impurities and moisture from penetrating into the fluorescent layer from outside.
However, in the thin film electroluminescent device whose insulating layer is relatively thin, the dielectric breakdown property is high, the reproducibility of a thin film insulating layer is reduced, and the fabrication process is complicated, due to an electric filed inputted from outside. Consequently, the problems of the thin film electroluminescent device have been solved, and a thick dielectric EL (TDEL) or solid state display device (SSD) having a good luminescence property have been developed.
FIG. 1 is a block diagram of a general solid state display device (SSD), in which a rear substrate 1 made of a predetermined material, a rear electrode 2 formed on the rear substrate 1, a thick dielectric layer 3 formed on the rear electrode 2 for preventing dielectric breakdown, a fluorescent layer 5 formed on the thick fluorescent layer 3 for generating visible rays, a thin dielectric layer 6 formed on the fluorescent layer 5, a transparent electrode 2 formed on the thin dielectric layer are staked one after another. In addition, a planarization layer 4 for flattening the interface between the thick dielectric layer 3 and the fluorescent layer 5 is further stacked between the thick dielectric layer 3 and the fluorescent layer 5.
Each of the layers constructing the structure of the above SSD will now be described in more detail.
The rear substrate 1 is generally made of an alumina material containing 96% aluminum oxide (Al2O3) because it must be sintered at a high temperature, and the rear electrode 2 is made of a conductive metallic material such as aluminum (Al) at a thickness of about 10 xcexcm being formed by vacuum deposition or screen printing method.
On the rear electrode 2, a dielectric material with a high permittivity is thickly formed, and thereby the thick dielectric layer 3 is stacked, in order to maintain a high dielectric breakdown property and a low driving voltage. Namely, the thick dielectric layer 3 is generally in the Perovskite crystal structure, and is formed by applying a paste powder mixture having a diameter of 2xcx9c3 xcexcm, which is mixed SrTiO3, PbTiO3, and BaTiO3 powder with an organic solvent, at a thickness of 50xcx9c200 xcexcm, and then baking the same at a temperature of 900xcx9c1000xc2x0 C. under oxidation atmosphere. Herein, in order to prevent the dielectric breakdown of the fluorescent layer 5, there must be no pinholes, the thick dielectric layer 3 must be strongly attached to the fluorescent layer 5, and the dielectric loss factor (tan xcex4) must be low.
On the thick dielectric layer 3, a fluorescent material is deposited at a thickness of 0.5xcx9c2 xcexcm using vacuum deposition method to form a fluorescent layer 5.
Herein, the fluorescent layer is made of ZnS:Sm material for emitting red visible rays, ZnS:Tb material for emitting green visible rays, and CaGa2S4:Ce material for emitting blue visible rays.
On the fluorescent layer 5, a thin dielectric layer 6 with a thickness of 1xcx9c3 xcexcm is formed, coated with Indium Tin Oxide (ITO) with a thickness of 0.5 xcexcm, and then baked for about one hour at a temperature of 450xcx9c500xc2x0 C. to form a transparent electrode 7.
Meanwhile, a planarization layer 4 is formed between the thick dielectric layer 3 and the fluorescent material 5 by applying PZT (lead zirconium titanate) with a thickness of a number of xcexcm using Sol-Gel or MOD (Metal Organic Decomposition) method, in order to smooth the surface of the thick dielectric layer 3.
The thusly formed SSD is sealed with a low melting glass or silicon sealant at a thickness of 10xcx9c20 xcexcm so that it is not contaminated by impurities or moisture from outside, thus becoming a finished product.
The driving principle of the thusly constructed SSD will now be described in brief.
First of all, when a predetermined voltage (e.g., 22V) is applied to the rear electrode 1 and the transparent electrode 7 electrons are emitted at the interface level of the thick dielectric layer 3 and the thin dielectric layer 6 adjacent to the fluorescent layer 5 by means of tunneling effect. The emitted electrons are accelerated by a high electric field (e.g., 106V/m) to become thermal electrons, said thermal electrons collide with atoms contained in the fluorescent material (e.g., ZnS:Mn) and thereby excite these atoms, said excited atoms emit visible rays while transiting to the ground state. With this principle, the solid state display displays desired images. At this time, the thick dielectric layer 3 serves to prevent dielectric breakdown and the diffusion between the rear electrode 2 and the fluorescent layer 5, stably supply a high voltage, and keep the solid state display""s own thermal stability.
Meanwhile, the method for fabricating a SSD will now be described.
(a) A rear electrode 2 of a predetermined shape is formed on a rear substrate 1 using an electrode printer. At this time, the rear electrode 2 is made of a conductive metal.
(b) A thick dielectric layer 3 is formed on the rear electrode 2 using screen printing method. At this time, the thick dielectric layer 3 has a thickness of preferably 100xcx9c300 xcexcm, and is made of SrTiO3, PbTiO3, and BaTiO3. As described above, the thick dielectric layer 3 formed at a predetermined thickness is baked at a temperature of 900xcx9c1000xc2x0 C. under oxidation atmosphere. Thus, the rear substrate 1 is made of aluminum oxide (Al2O3) capable of bearing a high baking temperature.
(c) A planarization layer 4 with a thickness of 2xcx9c3 xcexcm is formed on the thick dielectric layer 3 using Sol-Gel, screen printing, or spin coating method. At this time, the planarization layer 4 is made of a material such as PZT (Lead Zirconium Titanate)
(d) A fluorescent layer 5 with a predetermined thickness is formed on the planarization layer 4 using vacuum deposition method. At this time, the fluorescent layer 5 is made mainly of ZnS:Mn.
(e) A thin dielectric layer 6 with a thickness of 1xcx9c2 xcexcm is formed on the fluorescent layer 5 using vacuum deposition method.
(f) A transparent electrode 7 is formed on the thin dielectric layer 5 using vacuum deposition method. At this time, the transparent electrode 7 is made of Indium-Tin Oxide (ITO) which is a transparent material through which visible rays can be transmitted.
The solid state display device thusly fabricated must have the following characteristics in general.
Firstly, in order to reduce the driving voltage, the dielectric constant must be more than 1000.
Second, in order to keep a stability of the interface between the fluorescent layer and the dielectric layer, the dielectric strength must be more than 1.0xc3x97106V/m.
Thirdly, in order to keep a stability of the interface between the fluorescent layer and the dielectric layer, the surface roughness of the dielectric layer must be high.
Fourthly, in order to maintain a high dielectric strength, the amount of air bubbles must be minimized inside the dielectric layer, in the interface between the fluorescent layer and the dielectric layer, and in the interface between the electrode and the dielectric layer.
However, since the conventional solid state display is coated with thick dielectric and thereafter is baked at a high temperature of more than 1000xc2x0 C., a rear substrate capable of bearing the above temperature has to be used. Therefore, the material of the rear substrate used for the conventional solid state display is restricted to materials having a high melting point such as alumina.
In addition, since the conventional solid state display is baked at a high temperature, the electrode is damaged, pinholes are occurred due to impurities contained in vacuum, air bubbles are incompletely removed from the dielectrics and the interface thereof and thus the dielectric loss factor is increased, whereby the dielectric breakdown property is reduced and the adherence between the dielectric layer and the fluorescent layer is also reduced.
Accordingly, it is an object of the present invention to provide a thick dielectric composition for a solid state display whose thermal or electrical characteristics are improved.
It is another object of the present invention to provide a method for fabricating a solid state display in which the occurrence of pinholes due to impurities and the occurrence of air bubbles in the interface of dielectrics are restrained, and the adherence between the layers constructing the solid state display is increased, thus improving the dielectric breakdown property.
To achieve the above objects, in a thick dielectric composition for preventing dielectric breakdown of a solid state display, there is provided a first composition of a thick dielectric layer in accordance with the present invention which includes a P2O5xe2x80x94ZnOxe2x80x94BaO type glass, an oxide filler, and an organic solvent.
In a thick dielectric composition for preventing breakdown of a solid state display, there is provided a second composition of a thick dielectric layer in accordance with the present invention which includes a SiO2xe2x80x94ZnOxe2x80x94Ba2O3 type glass, an oxide filler, and an organic solvent.
In a thick dielectric composition for preventing breakdown of a solid state display, there is provided a third composition of a thick dielectric layer in accordance with the present invention which includes a PbOxe2x80x94ZnOxe2x80x94B2O3 type glass, an oxide filler, and an organic solvent.
A method for fabricating a solid state display in accordance with the present invention includes a step of forming a metal electrode on a predetermined substrate, and a step of forming thick dielectrics in green sheet or paste on the substrate having the above metal electrode and thereafter simultaneously heating and pressurizing the above thick dielectrics.